1. Technical Field of the Invention
The present invention relates to a two-dimensional code, and in particular, to a two-dimensional code having a rectangular region delineated by a plurality of cells and formed to have specific patterns for specifying the positions of cells in the rectangular region and distinguishing the code from the background.
2. Related Art
Various types of two-dimensional codes have been known and commercially used, which are composed of a plurality of cells arranged in a matrix form. In order to read the two-dimensional codes, a two-dimensional code reader is necessary. This reader usually captures a two-dimensional image data which contain a two-dimensional code, analyzes the image data to specify the area of the two-dimensional code (i.e., code area) in the image data, and decodes the image information of the specified code area.
As above, it is necessary to specify the area of a two-dimensional code in reading the two-dimensional code. Hence the two-dimensional code requires to be configured such that the code area can be specified quickly and accurately. With a view to meet this demand, there have been provided techniques disclosed by Japanese Patent Publication (Laid-open) No. 2-12579 and Japanese Patent Publication No. 2938338.
The technique disclosed by the former publication No. 2-12579 shows that an L-shaped specific pattern composed of continuous dark cells (square cells) arranged along two sides of a rectangle code area and a further specific pattern composed of dark cells and bright cells which are arranged alternately along the remaining two sides of the rectangular code area. In this way, since all the four sides of the rectangle code area are delimited by the specific patterns, it is easier for the reader to distinguish the code area from the background. However, in this case, it is required to use a large number of cells as the cells which should be assigned to the specific patterns, which are not data to be decoded and data for error correction. This leads to a reduction in the number of cells assigned to the data.
The latter reference No. 2938338 provides a technique, with which bright cells (white cells) and dark cells (black cells) are arranged in the data area, where the characteristics of the cells are changed under a given rule. Practically, a plurality of types of mask patterns (i.e., matrix patterns for changing the cell characteristics) are prepared in advance and, when producing a two-dimensional code, the respective mask patterns are superposed after data which has been arranged. After the superposition, an arrangement pattern showing the least continuity in the white cells and the black cells is adopted as an optimum arrangement pattern. This superposition technique makes it difficult for the bright cells and the dark cells to be continuously arranged at the borders of the code to the background, thus providing a higher distinction of the code area from the background. However, in the case of this technique, it is necessary to provide the code with information (mask pattern information) showing which type of mask pattern is adopted. Hence, it cannot help but to reduce the number of cells which can be assigned to both data to be decoded and data for error correction.